Human growth hormone (hGH) is multifaceted from both a functional and a chemical standpoint. The variety of different activities exhibited by hGH has been known for some time. More recently, it was recognized that monomeric hGH consists of several cmponents (isohormones B, C, D and E). The physiological significance of this chemical heterogeneity is not clear. Possibilities for their physiological role include: 1) Individual isohormones may exhibit selective functions in the spectrum of hGH actions. 2) Isohormone-B may represent a pituitary storage form or prohormone which is enzymatically activated before or during secretion. 3) One or the other isohormone may be secreted, depending on the body's need. It is proposed to investigate these possibilities by 1) studying the interaction of homogeneous hGH isohormones with GH-responsive tissues in terms of receptor binding and biological response; 2) studying the isohormone nature of circulating and urinary hGH; 3) assessing the contribution of peripheral tissues to isohormone generation by in vitro and in vivo studies.